I've just installed Omnisphere 2 on Cubase Pro 9.5 64-bit version (9.5.30 On Windows 10) and Omnisphere is blacklisted VST 2 instrument. I went into Cubase plugin manager in blacklisted section and tried to 'reactivate' but Cubase just crashes.

plug-in manager\vst instruments panel\settings, where there's a re-sync/re-scan to press. Once I did that the Omnishpere.dll in the blacklist went from vst 2 to being in the VST instruments list as vst 2.4. Now everything works!